Production of deposits by cathode sputtering

ABSTRACT

The invention eliminates the drawbacks of the prior methods for cathode-sputtering deposition by providing basically the use of a substantially neutral plasma adapted to be carried in the form of a stable parallel beam to a substantial distance from the source without resorting to intricate electrode arrangements.

United States Patent [72] Inventors Jean Jacques Bessot Arpajon; JeanClaude Burlurut, Versailles, both of France [21] Appl. No. 739,029

[22] Filed June 21, 1968 [45] Patented Oct. 26, 1971 [73] AssigneeSociete Anonymey, Societe Alsacienne de Construction Atomiques deTelecommunications et dElectronique Alcatel Paris, France [32] PriorityJune 22, 1967 3 3] France [541 PRODUCTION OF DEPOSITS BY CATHODESPUTTERING 3 Claims, 2 Drawing Figs. [52] US. Cl... 204/298 [51] Int.CL... C23c 15/00 [50] Field of Search [56] References Cited UNITEDSTATES PATENTS 3,436,332 4/1969 Oda et a1. 204/298 3,409,529 11/1968Chopra et al. 204/298 3,393,142 7/1968 Moseson 204/298 3,507,774 4/1970Muly 204/298 3,516,919 4/1970 Gaydou et al 204/298 FOREIGN PATENTS1,102,175 2/1968 Great Britain 204/298 Primary Examiner-John H. MackAssistant ExaminerSidney S. Kanter Attorney-Littlepage, Quaintance, Wray& Aisenberg ABSTRACT: The invention eliminates the drawbacks of theprior methods for cathode-sputtering deposition by providing basicallythe use of a substantially neutral plasma adapted to be carried in theform of a stable parallel beam to a substantial distance from the sourcewithout resorting to intricate electrode arrangements.

'IIIIII/III/IM PRODUCTION OF DEPOSITS BY CATIIODE SPUTTERING BACKGROUNDOF THE INVENTION The invention relates to improvements to the methodwhereby a deposit in thin-layer form can be obtained by cathodesputtering. The invention also relates to the devices for carrying suchimprovements into effect.

lt-has for long been proposed to use the cathode sputtering phenomenonfor producing deposits onto objects or substrates.

The most ancient methods of this type consisted in using a cathode modeof the material to be sputtered and in depositing said material onto ananode through a conducting medium consisting of an inert gas which wasionized by the electrons travelling from the cathode to the anode. Suchmethods involved high power consumption since they used a cold cathode,so that voltages of several thousand volts were required to producesatisfactory electron flow between the cathode and anode. Moreover, thistype of discharge required a rather high gas pressure (from 10 to 100microns Hg).

An initial improvement to the prior art, which afforded a reduction inthe power necessary to provide the ion flow, was the use of a hotcathode. As a matter of fact, an electron-accelerating voltage of lessthan 100 volts across the anode and cathode is then sufficient forextraction of the electrons. In such a system, the source ofgas-ionizing electrons and the source of material to be sputtered aredistinct. The material being sputtered is removed from a target which isbombarded by the ions formed between the cathode and anode, said ionsbeing accelerated by said target.

To further increase the electron density within the gas to be ionized, afurther prior art proposal was to place a tubular anode in front of afilament heated to a high temperature and to establish within thetubular anode a magnetic field parallel to the axis of said tubularanode, the electrons and ions formed being then caused to follow helicalpaths around the electrode axis, while a still further increase in thedegree of gas ionization was provided by locating near the tubularelectrode a second electrode of annular shape, also called extractionelectrode, at a negative potential with respect to the tubularelectrode. This extraction electrode acts to repell the electronstowards the tubular electrode and to direct the ions towards the targetto be bombarded. The result is an excellent electron concentration inthe interior of the tubular electrode and a high-density electron flowat the outlet of the second electrode. However, the resulting ion fluxtends to be divergent and further electrodes must be so arranged as torender it parallel or convergent. Consequently, the devices for carryingsuch a method into effect become intricate, all the more as the voltagesand relative positions of the electrodes are to be carefully controlled.

SUMMARY OF THE INVENTION The purpose of the invention is to eliminatethe drawbacks of the prior methods for cathode-sputtering deposition,mainly by providing the use of a substantially neutral plasma which canbe carried in the form of a stable parallel beam to a rather greatdistance from the source without resorting to intricate electrodearrangements.

According to the invention, the method for producing a thin-layerdeposit onto an object by cathode sputtering, the material to besputtered being taken from a target, consists basically in establishingin vacuo at some distance from said target a substantially neutralplasma in pencil form by ionizing an inert gas by means of adiaphragm-delineated beam of electrons issuing from a heated filamentand by subjecting said electrons to the concurrent action of an electricfield which is created by a tubular anode located adjacent saiddiaphragm, and of a magnetic field which extends parallel to said anodeaxis, and in so arranging said target, which is at a negative potential,that said plasma pencil will lie level with the surface thereof, saidobject to be coated standing in close vicinity and opposite to saidsurface.

An advantageous feature of the method of the invention is that thesubstantially neutral plasma can be formed. within one compartment ofthe vacuum chamber and the target be arranged in another compartment,which may be under higher vacuum.

It is easy to extract the positive ions fromthe plasma and to acceleratethese for directing them onto the target. by bringing said target to anegative potential of some hundreds volts.

Part of the electrons emanating from the filament and passing throughthe diaphragm aperture are not confined within the tubular anode andbecome mingled with the positive ion flow, e.g. argon ion flow, thusproviding outside the.tubular electrode a substantially neutralplasmacomprised of, -f

filament and the tubular anode, which is at a positive potential withrespect to the filament, and of the magnetic field which lies parallelto the ,tubular anode axis and is advantageously I generated by a coilexternal to the vacuum chamber.

When the method according to the invention is carried into effect in avacuum chamber including two compartments, then the compartments wherethe plasma is formed is connected to the target-containing compartmentthrough an aperture of a low size sufficient to allow free passage ofthe plasma beam.

Thus, the invention affords operational facilities which are an advanceover the prior art According to the invention, the rate of depositioncan reach a few tens of microns per hour, while the prior methodsallowed but for deposition rates of some 23 u/hour.

The material used for sputtering may be silver, nickel, titanium,tantalium or other materials useful for thin-layer deposition. Theobjects to be coated may be any articles especially insulating parts forthe production of thin-layer circuits. The inert gas used to form theplasma is advantageously argon.

BRIEF DESCRIPTION OF THE DRAWINGS Two devices for carrying the method ofthe invention into effect will now be described by way of examples with,reference to the accompanying drawings which are diagrammatic views inaxial section, of which:

FIG. 1 shows the first device;

FIG. 2 shows the second device, which is to be preferred in practice.

DETAILED DESCRIPTION In FIG. 1, there is shown at l a vacuum vessel andreference 2 designates a filament which is brought by resistance heatingthrough wires 20, 2b to a temperature of 2,000-2,800 C. F ilament 2 isarranged within a bent tube 3, cooled by water circulated through 4 andprovided in its upper portion with a diaphragm having a central hole 5.An anode6, fed through 6a, consists of a copper tube open at both endswhich, as illustrated, has a diameter of 90 mm. and a height of l5 0 mm.A coil 7 coaxial with the anode tube 6 creates in said tube a magneticfield of 200-500 gauss. A target 8, made of the material to besputtered, is suspended by means over the anode tube 6, lying paralleland relatively close to the anode tube axis. Oil circulated through 9acts to cool the targetwhen the targetforming material cannot withstandthe temperatures which may be attained by said target. The plate 10 tobe coated is disposed on a support I] of stainless'steel which isdirectly connected to earth at Ma. An inlet valve 12 serves to admit aninert gas such as argon, which flows into vessel 1 through duct 120. Avacuum unit, not shown, is connected to the vacuum vessel through a tube13.

In this device, electrons are emitted by filament 2 which is heated to atemperature of 2,500 C. Said electrons are directed by tube 3 to theanode 6, which is at a positive potential ranging from 50 to I00 volts.The magnetic field of coil 7.

which ranges from 100 to 500 gauss and is parallel to the axis of anode6, repels the electrons towards the anode axis by causing them to whirlaround said axis. Argon coming in through valve 12 at a pressure from lto 10 torr is forced through the anode tube 6 and is ionized by theelectrons from tube 3. Since the electrons are confined within theanode, the amount of argon molecules being ionized per unit volume isconsiderably increased. A 25 percent ionization ratio may be reached.

The anode current is of the order of 9 a. The ion current is of theorder of 300-500 ma. across a target of 25 cm. area, Le. a currentdensity of l2-20 ma./cm. The ion flow is attracted by the target, whichis at a negative potential of 800 volts. When the material to besputtered is insulating, the voltage used on the target is caused to bealternatively positive and negative at a frequency from 13 to 20 Ml-lz.,so that the target under sputtering by the ion bombardment beneutralized by the electron bombardment.

Since the ion flow is concentrated near the anode tube axis, the targetis arranged close and parallel to said axis so as to be at the highestion density location. Usually, the part to be coated or workpiece isspaced from the target by less then cm., so that the average free travelof the gas molecules is higher than said spacing and that the sputteredparticles are not driven outside the space between the target and theworkpiece.

With this arrangement, the rate of deposition can be five to timeshigher than that obtained with the usual arrangements.

FIG. 2 shows an embodiment of the same device affording better handlingfacilities.

The vacuum vessel is formed of two compartments l4, 15 which can beconnected through a coupling 16. Compartment [4, which is cooled througha coil 17, contains the filament fed by wires 18a, 18b and a tubularelectrode forming the anode 19, which is fed at 19a. A diaphragm 22,with an axial aperture 22a, is arranged in front of filament 18. Amagnetic coil 20 or a tubular magnet is coaxial with the anode tube 19.A valve 21 serves to admit an inert gas or any other gas through duct21a. Compartment 15 is closed at its upper portion by a removable cover15a. Within compartment 15 are a target 23 fed at 23a and cooled by oilor air circulated through 24 and a support 25 for the object 26 to becoated. A vacuum unit not shown is connected to compartment 15 by a tube27.

The values of the various parameters, viz, filament temperature, anodevoltage, anode current, ion current density, target voltage, magneticfield, are of the same order as in the case of the device in FIG. 1.

It should be noted that, for the two devices illustrated, theseparameters can vary within the following ranges:

target voltage magnetic field ll up to 500 gnuss The device in FIG. 2permits to increase the travel of the neutral plasma flow emerging fromthe anode tube and to coat objects of reasonable size. For this purpose,compartments l4, 15 are made separable so as to be more easy toconstruct. Compartment 14 has a length substantially equal to the lengthof the plasma flow travel. Compartment 15a is of sufficient capacity toaccommodate objects of relatively great extent lying on support 25.

This device provides the following advantages: on the one hand, the coilcreating the longitudinal magnetic field can be placed closer to thetubular electrode axis and, on the other hand, higher vacuum may beobtained in the sputtering chamber than in the portion where the plasmais formed. Thus, optimum sputtering conditions are afforded by themaintenance within the tubular electrode of the best environment for theformation of a highdensit lasma. We claini:

1. Sputtering apparatus for depositing thin layers on substrates byion-bombarding a target and sputtering material from the target to asubstrate comprising: a vacuum chamber, evacuating means connected tothe chamber for reducing pressure therein, an electron beam sourcemounted in the chamber comprising a filament, a diaphragm substantiallycovering the filament and having a central opening through which anelectron beam projects, an elongated hollow cylindrical anode on anopposite side of the diaphragm from the filament and encircling theelectron beam and extending substantially along an axis of the electronbeam as it emerges through the diaphragm opening, ion beam generatingmeans including said cylindrical anode and an electromagnetic coilsurrounding said cylindrical anode, and gas injection means connected tothe vessel and positioned therein for flowing gas into the cylindricalanode, first power supply means connected to the cylindrical anode forpositively biasing the cylindrical anode and second power supply meansconnected to the coil for energizing the coil, whereby the cylindricalanode pulls electrons through the opening in the diaphragm whereuponplasma having positively charged ions is formed, whereby the cylindricalanode radially focuses positive ions, forming along its axis an ionbeam, and whereby the magnetic coil propels the ion beam along the axisaway from the diaphragm opening, a target mounted in the vessel on oneside of the ion beam on an opposite end of the cylindrical anode fromthe diaphragm, a third power source connected to the target fornegatively biasing the target and attracting ions to a surface thereofcontaining material to be sputtered, and a substrate mounted in thevessel adjacent the ion beam and having a surface to be coatedpositioned opposite the target for receiving material sputtered from thetarget.

2. The apparatus of claim 1 wherein filament comprises a helical wirehaving an axis and wherein axes of the filament, diaphragm opening,cylindrical anode and electromagnetic coil are concurrent.

3. The apparatus of claim 1 wherein the vacuum vessel comprises a mainchamber and a tube opening into the main chamber and extending outwardfrom an open connection therewith, wherein the filament, diaphragm andcylindrical anode are positioned within the tube and wherein thediaphragm opening, cylindrical anode have axes concurrent with a centerof the open connection.

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2. The apparatus of claim 1 wherein filament comprises a helical wirehaving an axis and wherein axes of the filament, diaphragm opening,cylindrical anode and electromagnetic coil are concurrent.
 3. Theapparatus of claim 1 wherein the vacuum vessel comprises a main chamberand a tube opening into the main chamber and extending outward from anopen connection therewith, wherein the filament, diaphragm andcylindrical anode are positioned within the tube and wherein thediaphragm opening, cylindrical anode have axes concurrent with a centerof the open connection.